1. Field of the Invention
This invention relates to electrochemical capacitor electrodes comprised of materials from hydrous ruthenium oxide, other hydrous metal oxides, or hydrous mixed metal oxides. More particularly, the invention relates to such electrodes comprised of hydrous ruthenium oxide, or other hydrous metal oxides, or hydrous mixed metal oxides and to electrochemical capacitors or electrochemical power sources comprising such electrodes. This invention also relates to a method for the preparation of fine powders or thick films of such materials at low temperatures.
2. Prior Art
Electrochemical capacitors (EC's) are devices which store electrical energy at the interface between an ionically-conducting electrolyte phase and an electronically-conducting electrode material. EC's were first described in a 1957 patent by Becker. The first practical devices were pioneered by SOHIO as described in U.S. Pat. No. 3,536,963 based on the double-layer capacitance developed at the interface between high-area carbon electrodes and sulfuric acid electrolyte solution. A complementary system, but originating from a different electrochemical phenomenon, that is development of pseudocapacitance associated with a surface reaction, was developed by Conway in 1975, in collaboration with Continental Group, Inc. See Can. Pat. by Craig which is listed infra. The materials possessing pseudocapacitance discovered in Conway et al.'s work are metal oxides which include ruthenium oxide (RuO.sub.2), iridium oxide (IrO.sub.2), cobalt oxide (CoO.sub.2), molybdenum oxide (MoO.sub.2), and tungsten oxide (WO.sub.3). The most effective material discovered was RuO.sub.2 which gives a reversibly accessible pseudocapacitance of many Farads per gram over a 1.4 V range.
Heretofore, RuO.sub.2, has been fabricated by the thermal decomposition of ruthenium chloride or hydrous ruthenium chloride. RuO.sub.2, like other dioxides of the platinum group, e.g. RhO.sub.2, OsO.sub.2, and IrO.sub.2, exhibits metallic conductivity and possesses the rutile structure. The pseudocapacitance, which arises at the RuO.sub.2 and the electrolyte interface, is due to the facile ionic species absorption on the surface of the RuO.sub.2 electrode material.
In order to maximize the charge or energy storage per unit weight of oxides in this type of system, it is desirable to maximize the surface area of the electrode material. Such a maximum BET surface area of 130 m.sup.2 /gram was achieved by Raistrick for optimized processing.
The observed capacitance per unit mass (F/g) and the observed capacitance per unit area (F/cm), which are determined from the measured electrochemical capacitance, the measured surface area, and the known amount of RuO.sub.2 present in the electrode, are 380 F/g and 200-300 .mu.F/cm.sup.2, respectively, in a 1 V range in sulfuric acid electrolyte. Based on the assumption that one H may be adsorbed on each exposed O atom, a charge density of 200 .mu.C/cm.sup.2 is estimated by Raistrick. This suggests that the observed capacitance 380 F/g is the maximum value that can be achieved for RuO.sub.2.
These and other references, which are relevant to the present invention, include Can. Pat. No. 1,196,683 issued in 1985 to Craig; U.S. Pat. No. 2,800,616 issued in 1957 to Becker; and U.S. Pat. No. 3,536,963 issued in 1970 to Boos; Conway, Journal of the Electrochemical Society, vol.138, pp. 1539-15, 1991; Raistrick, Proceedings of First Conference on Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla., Dec. 9-11, 1991, Ansum Enterprises Inc., Boca Raton, Fla.; and U.S. Pat. No. 5,003,428, issued to Shepherd on Mar. 26, 1991.